In recent years, the depletion of fossil energy sources such as oil and coal has been a problem. Another major problem is destruction of environment such as global warming that is attributable to an increase in emissions of CO2, which is generated as the fossil fuels are burned. In such a situation, solar power generation has been put to practical use as an alternative source of clean energy that utilizes unlimited amounts of solar radiation energy. Solar cells are an essential part of a solar power generation system that converts solar energy directly into electricity, and are made from photovoltaic devices such as crystalline silicon, polycrystalline silicon, amorphous silicon, copper indium selenide, and compound semiconductors. As for a structure thereof, a single photovoltaic device is not used without being changed. In general, several, or dozens of, photovoltaic devices are wired together in series or parallel. Moreover, packaging is carried out in various ways to protect the devices for a long period of time. In this manner, the devices are turned into units as solar cell modules.
The solar cell modules include the following basic functions: sending solar radiation energy to the photovoltaic devices in an efficient manner, and protecting the photovoltaic devices and internal wiring from harsh natural environment for a long period of time. In a solar cell module, in general, the following are stacked in the order described below: an upper transparent material, which includes a glass of a plane which sunlight hits, transparent plastics, and the like; an sealing-material layer, which includes thermoplastic resin such as copolymer of ethylene and vinyl acetate (referred to as EVA, hereinafter); a plurality of solar cells, which function as photovoltaic devices; a sealing-material layer, which is similar to the above sealing-material layer; and a solar cell backsheet. The above components are formed integrally by vacuum heating lamination method or the like.
The solar cell backsheet is required to be excellent in mechanical strength to protect internal components such as solar cells and leads, as well as to be excellent in various characteristics such as weather resistance, heat resistance, water resistance, light resistance, and chemical resistance. In particular, the solar cell backsheet is required to have a high level of gas barrier property in order to prevent ingress of moisture, oxygen, and the like. What is important to keep the barrier property is a material having barrier property, as well as adhesion and adhesiveness stability with the sealing-material layer such as EVA. The reason is that separation and discoloration of the sealing-material layer and corrosion of the wiring could occur due to transmission of moisture from an interface, possibly having an impact on the output of the modules. Furthermore, in order to contribute to improving power generation efficiency, an inner surface is required to be white.
Conventionally, for the solar cell backsheet, fluororesin has been used: the fluororesin is good in weather resistance and flame resistance, as well as good in adhesiveness to EVA that is frequently used as filler, such as polyvinyl fluoride (PVF) or polyvinylidene fluoride (PVDF). However, the use of a single sheet of fluororesin comes with problems in water-vapor barrier properties, transparency, weather resistance, flame resistance, and the like.
To solve the above problems, the use of polyester-based film as a solar cell backsheet is frequently proposed. For example, what is disclosed in Patent Document 1 is a backside sealing film for solar cell, which includes a laminated body of polybutylene terephthalate (PBT) film containing titanium oxide produced by chlorine method. However, while showing an improvement in hydrolyzability compared with PET and balanced physical properties, the backside sealing film has the problem that the backside sealing film is poor in adhesiveness to the sealing-material layer such as EVA. As described above, the use of conventional techniques cannot lead to a solar cell backsheet that is good in adhesiveness to the sealing-material layer made of EVA, and excellent in water-vapor barrier properties, transparency, weather resistance, and flame resistance.